Coal-fired combustion systems such as those used in large utility applications require finely-ground coal particles or "fines" for efficient operation. In general, it is desirable to use only very-finely pulverized coal in such systems in order to keep NOX emissions and oversized loss-on-ignition (LOI) unburned coal particles from contaminating the marketable ash byproduct of the combustion chamber. It is accordingly important to maintain close control over the fineness of the pulverized coal fed into the combustion system.
Bowl mill-type pulverizers, such as the type disclosed in U.S. Pat. No. 4,687,145, are commonly used to grind the coal and classify the resulting fines. A vertical feedpipe drops raw coal from several feet adore the pulverizer to the center of the pulverizer or grinding. An annular and upwardly-directed flow of air through a ring-shaped "throat" blows the ground coal particles up and around the pulverizer to a classifier system and combustion delivery chute feeding the combustion chamber. The classifier system removes oversized particles of coal from the flow of air and coal fines, returning them to the pulverizer for regrinding.
A known system for classifying these upwardly traveling fines consists of an inverted classifier cone mounted above the pulverizer and concentric with the feedpipe that delivers raw coal to the center to the pulverizer. The lower, smaller outlet end of the classifier cone essentially surrounds the outlet end of the feedpipe, while the larger, upper inlet or mouth of the cone surrounds the combustion delivery chute.
A stationary ring of classifier vanes is mounted at the mouth of the cone to receive the annular, upward flow of pulverized coal/air from the pulverizer and redirect it into the classifier cone in a centrifugal As the coal fines and air swirl around in the classifier cone, the heavier particles gravitate to the sides and settle out at the bottom of the cone, while the lighter, more finely ground fines are swirled up and into the entrance of the combustion delivery chute.
As the heavier particles of coal collect at the bottom of the classifier cone, they are typically contained by a flapper valve assembly at the bottom of the cone, comprising a series of vertically hanging plates blocking the openings of one or more outlet chutes. The plates are relatively heavy, and are forced open only intermittently by the weight of the accumulated coal at the bottom or the classifier cone. These fine "rejects" then fall into the bowl mill pulverizer along with incoming raw coal from the feedpipe for regrinding.
There are a number of disadvantages inherent in prior art systems such as those described above.
The prior art positioning of the feedpipe ant classifier cone outlets well above the pulverizer often results in fine rejects being blown back up through or around the classifier cone when the flapper assembly opens for a discharge. This is primarily due to the position of the outlets relative to the annular flow of coal fines/air from the pulverizer
Moreover, the flapper assembly and other prior art intermittent cone discharge systems such as "hula skirt" assemblies (circular arrangements of overlapping metal leaves) can become stuck in an open position, adding to the problem of fine reject backflow into the combustion delivery chute and further defeating the function of the classifier cone.